Electrical generators convert mechanical energy into electrical energy by rotating an electrical conductor (rotor) with respect to a relatively stationary magnetic element (stator), which produces an electric current in the rotor. Rotational motion may be imparted to the rotor by a variety of means, such as an electric motor, wind turbine or the like. Electrical generators can be used to generate both DC and AC current. Within the class of AC current generators, also known as alternators, is the so-called "polyphase" AC generator, in which both the stator and rotor are equipped with polyphase electrically conductive windings.
The output frequency of an alternator is proportional to the speed of rotation (RPM) of the rotor. For example, a three phase AC generator may require a rotor speed of 1800 RPM to generate a 60 Hz AC signal. Some electrical equipment which operates on AC current requires constant frequency (e.g. 60 Hz) for efficient operation. It is therefore critical to maintain a substantially constant rotor speed for constant frequency output. It is also critical to maintain a constant rotor speed to reduce wear and tear on mechanical components and for optimum operating efficiency of the machine. Machinery, such as electrical generators, are dynamically balanced to run most efficiently at one or more selected speeds of rotation.
Wind-driven propellers or turbines are often used to supply mechanical energy to turn the alternator rotor. One problem associated with such wind-driven machines is that wind speed variations will produce corresponding variations in the rotor speed and output frequency unless the rotor speed is otherwise regulated.